1. Field of the Invention
The present invention relates to a neutral control device of an automatic transmission of a motor vehicle, which controls a hydraulic pressure so that a forward drive engaging element has no engaging capacity, i.e., the forward drive engaging element is placed in a neutral state, when neutral control conditions that the vehicle is stopped and an accelerator pedal is released are satisfied while a forward drive range is being selected.
2. Description of the Prior Art
A known example of the neutral control device of the automatic transmission is disclosed in Japanese Patent Application laid-open Publication No. 4-290671.
This known neutral control device was developed in an attempt to improve the response to cancellation of control for preventing creeping of the vehicle, and the durability of a frictional engaging device, while at the same time reducing shift shocks. To this end, in a system adapted to prevent creeping of the vehicle when certain conditions indicating a neutral state of the vehicle are satisfied with a forward drive range being selected, and smoothing control of an engaging pressure is performed upon cancellation of the creeping preventive control, a forward clutch is engaged in an early time when cancellation of the creeping preventive control is to be followed by quick start of the vehicle, and the smoothing control is performed as intended when cancellation of the creeping preventive control is to be followed by slow start of the vehicle.
In the known neutral control device of the transmission, smoothing control is performed when the vehicle is to be slowly started after the creeping preventive control is canceled. Under this smoothing control, however, the forward clutch pressure cannot be controlled as desired if only a solenoid Sd for supplying a hydraulic pressure to the forward clutch is actuated or operated for executing this control while the line pressure is varied in a normal manner in accordance with changes in the position of the accelerator pedal.
Namely, the line pressure serves as an original pressure of the frictional engaging device, and therefore the frictional engaging pressure to be applied to the forward clutch, or the like, is increased with an increase in the line pressure. If the forward clutch pressure is subjected to smoothing control upon slow start of the vehicle, therefore, the obtained forward clutch pressure becomes equal to a hydraulic oil pressure as a sum of a direct control pressure and the line pressure, as shown in FIG. 11, resulting in increased shocks upon engagement of the forward clutch, thus causing undesirable shocks upon start of the vehicle.